Normal cardiac rhythm template generation system and method

ABSTRACT

A method and system provides for generating a snapshot representative of one beat of a patient&#39;s normal cardiac rhythm. Cardiac rate channel signals and shock channel signals are sensed. A fiducial point is determined for a predefined number of the cardiac rate channel signals. A predefined number of the shock channel signals are aligned using the fiducial point. A template is generated using the aligned shock channel signals, whereby the template is representative of one of the patient&#39;s normal supra-ventricular conducted cardiac beats. The template is updated on a periodic basis.

FIELD OF THE INVENTION

[0001] The present invention relates generally to implantable medicaldevices and, more particularly, to generating, with an implantablemedical device, a template representative of one beat of a patient'snormal cardiac rhythm.

BACKGROUND OF THE INVENTION

[0002] Proper cardiac function relies on the synchronized contractionsof the heart at regular intervals. When normal cardiac rhythm isinitiated at the sinoatrial node, the heart is said to be in sinusrhythm. However, when the heart experiences irregularities in itscoordinated contraction, due to electrophysiologic disturbances causedby a disease process or from an electrical disturbance, the heart isdenoted to be arrhythmic. The resulting cardiac arrhythmia impairscardiac efficiency and can be a potential life threatening event.

[0003] Cardiac arrhythmias occurring in the atria of the heart, forexample, are called supra-ventricular tachyarrhythmias (SVTs). SVTs takemany forms, including atrial fibrillation and atrial flutter. Bothconditions are characterized by rapid, uncoordinated contractions of theatria. Besides being hemodynamically inefficient, the rapid contractionsof the atria can also adversely effect the ventricular rate. This occurswhen the aberrant contractile impulse in the atria are transmitted tothe ventricles. It is then possible for the aberrant atrial signals toinduce ventricular tachyarrhythmias.

[0004] Cardiac arrhythmias occurring in the ventricular region of theheart, by way of further example, are called ventriculartachyarrhythmias. Ventricular tachycardia (VTs), for example, areconditions denoted by a rapid heart beat, 150 to 250 beats per minute,that has its origin in some abnormal location with the ventricularmyocardium. The abnormal location typically results from damage to theventricular myocardium from a myocardial infarction. Ventriculartachycardia can quickly degenerate into ventricular fibrillation (VF).Ventricular fibrillation is a condition denoted by extremely rapid, nonsynchronous contractions of the ventricles. This condition is fatalunless the heart is returned to sinus rhythm within a few minutes.

[0005] Implantable cardioverter/defibrillators (ICDs) have been used asan effective treatment for patients with serious ventriculartachyarrhythmias. ICDs are able to recognize and treat tachyarrhythmiaswith a variety of tiered therapies. These tiered therapies range fromproviding anti-tachycardia pacing or cardioversion energy for treatingventricular tachycardia to defibrillation energy for treatingventricular fibrillation. To effectively deliver these treatments, theICD must first identify the type of tachyarrhythmia that is occurring,after which appropriate therapy is provided to the heart. In order toapply the proper therapy in responding to an episode of tachyarrhythmia,the ICD may compare sensed cardiac signals to a previously stored normalsinus rhythm (NSR) signal waveform. It is appreciated that the storedNSR signal waveform must accurately characterize a patient's true normalsinus rhythm in order to properly identify potentially fatal deviationsfrom normal cardiac activity.

[0006] For the reasons stated above, and for other reasons stated belowwhich will become apparent to those skilled in the art upon reading thepresent specification, there is a need in the art for reliably andaccurately characterizing a patient's normal cardiac rhythm. Thereexists a further need for such an approach that is adaptive andaccommodates changes in the patient's normal cardiac rhythm over time.The present invention fulfills these and other needs.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a method and system forgenerating a snapshot representative of one beat of a patient's normalcardiac rhythm. In accordance with one embodiment of the presentinvention, rate channel signals and shock channel signals are sensed. Afiducial point for the rate channel signals is determined. The shockchannel signals are aligned using the fiducial point. A template isgenerated using the aligned shock channel signals. The template isrepresentative of one of the patient's normal supra-ventricularconducted beats. The template may be updated on a periodic basis, suchas several times per day.

[0008] Using subsequently detected beats, confirmation processes arecarried out prospectively to confirm that the generated template isrepresentative of one of the patient's normal supra-ventricularconducted beats. According to one approach, a confirmation process usessubsequently detected template beats to determine whether the generatedtemplate is or is not representative of one of the patient's normalsupra-ventricular conducted beats.

[0009] One confirmation process involves determining that no template ispresently stored,.and, in response to confirming that each of a numberof subsequently detected template beats correlates with the generatedtemplate, storing the generated template. Another confirmation processinvolves determining that no template is presently stored, anddiscarding the generated template in response to confirming that each ofa number of subsequently detected template beats fails to correlate withthe generated template. A further confirmation process involvesdetermining that a template is presently stored, and retaining thestored template in response to determining that each of a number ofsubsequently detected template beats correlates with the storedtemplate.

[0010] Another confirmation process involves determining that a templateis presently stored, generating a new template in response to confirmingthat each of a number of subsequently detected template beats fails tocorrelate with the stored template, and replacing the stored templatewith the new template in response to confirming that each of a number ofnewly detected template beats correlates with the new template. Afurther confirmation process involves determining that a template ispresently stored, generating a new template in response to theconfirming that each of a number of subsequently detected template beatsfails to correlate with the stored template, and retaining the storedtemplate and discarding the new template in response to confirming thateach of a number of newly detected template beats fails to correlatewith the new template.

[0011] The template generation methodology typically involves averagingor median filtering the aligned shock channel signals. For example,averaging the aligned shock channel signals involves point-by-pointaveraging or median filtering of n samples acquired from the same timelocation of aligned n template beats. The template generationmethodology also involves determining that the rate channel signalssatisfy predefined normalcy criteria using a running average (RRavg) ofa number of RR intervals.

[0012] For example, after initiating template updating, a runningaverage (RRavg) of a number of RR intervals is compared to apredetermined rate threshold. If RRavg is less than a predeterminedinterval, template updating is suspended. By way of further example, abeat is classified as a regular beat if an RR interval associated withthe beat falls within a predetermined percentage range of RRavg.Further, a heart rate is classified as regular if a predeterminedpercentage of the beats are regular beats.

[0013] Template generation may also involve skipping processing of asubsequently sensed rate channel signal if the subsequently sensed ratechannel signal is detected before processing of a current sensed ratechannel signal is completed. The rate channel is also monitored fornoise. If the rate channel is determined to be noisy, the beat measuredfrom the noisy rate channel is classified as a noisy beat.

[0014] An automatic gain control (AGC) operation of the templategeneration methodology involves computing an average peak amplitude of anumber of beats. The shock channel gain is adjusted to an available gainthat sets the average peak amplitude to a predetermined percentage of amaximum ADC (analog-to-digital converter) value, such as 60% of themaximum ADC value.

[0015] According to further template generation operations, sensed beatsare classified as NSR beats in response to satisfying a first set ofcriteria. NSR beats are classified as template beats in response tosatisfying a second set of criteria. Generating the template, accordingto this embodiment, includes generating the template using the alignedtemplate beats.

[0016] The fiducial point to which the shock channel template waveformsare time aligned is characterized by a fiducial point type. The fiducialpoint type is determined by determining the larger of a positive peakand a negative peak for each of a number of NSR beats. The fiducialpoint type for alignment is determined by determining whether themajority of NSR beats have positive peaks or negative peaks. Aligningthe shock channel signals involves aligning shock channel waveforms oftemplate beats centered with respect to the fiducial point. A templatewaveform is generated by averaging a predetermined number of the timealigned template beats.

[0017] Generating the template further involves determining a number offeatures of the template. The template features include an absolutemaximum peak and at least one of a turning point and a flat slope point.

[0018] A body implantable system preferably implements a templategeneration methodology of the present invention. The body implantablesystem is disposed in a housing having a can electrode. A lead systemextends from the housing into a heart and includes electrodes. Adetector system, coupled to the lead system, detects rate channelsignals and shock channel signals sensed by one or both of the leadsystem electrodes and the can electrode. A control system, whichincludes a controller and a tachyarrhythmia detector/template generator,is coupled to the detector system. The control system determines afiducial point for the rate channel signals, aligns the shock channelsignals using the fiducial point, and generates a template using thealigned shock channel signals. The control system performs otheroperations, such as those discussed above, as part of a templategeneration methodology of the present invention. For example, thecontrol system updates the template periodically. By way of furtherexample, the control system updates the template in response todetecting establishment of connectivity between the lead system and thedetector system.

[0019] The above summary of the present invention is not intended todescribe each embodiment or every implementation of the presentinvention. Advantages and attainments, together with a more completeunderstanding of the invention, will become apparent and appreciated byreferring to the following detailed description and claims taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a partial view of one embodiment of an implantablemedical device with an endocardial lead system extending into atrial andventricular chambers of a heart;

[0021]FIG. 2 is a block diagram of a cardiac defibrillator with which atemplate generation methodology of the present invention is implemented;

[0022]FIG. 3 illustrates a number of steps associated with shock channeltemplate generation in accordance with an embodiment of the presentinvention;

[0023]FIG. 4 illustrates various steps associated with shock channeltemplate generation in accordance with an embodiment of the presentinvention;

[0024]FIG. 5 is a more detailed illustration of various steps associatedwith shock channel template generation in accordance with an embodimentof the present invention;

[0025]FIG. 6 illustrates details of the various steps shown in FIG. 5 ina linear time fashion;

[0026]FIGS. 7 and 8 respectively illustrate positive and negative typefiducial points determined from rate channel signals in accordance withthe principles of the present invention;

[0027]FIG. 9 illustrates alignment of shock channel waveforms withrespect to a fiducial point determined from rate channel signals inaccordance with the principles of the present invention;

[0028]FIGS. 10 and 11 show morphological features, including turningpoint and flat slope features, respectively, selected in accordance withthe principles of the present invention;

[0029] FIGS. 12A-12B illustrate shock and rate waveforms generated inaccordance with the principles of the present invention;

[0030] FIGS. 13A-13B illustrate shock and rate waveforms of the presentinvention, with the shock template including a number of selectedmorphological features;

[0031] FIGS. 14A-14B illustrate shock and rate waveforms of the presentinvention, with amplitude and location information associated withselected morphological features;

[0032]FIG. 15 illustrates generation of a shock channel template by useof a number of averaged template beats aligned with respect to a ratechannel fiducial point established in accordance with an embodiment ofthe present invention; and

[0033]FIG. 16 illustrates the resultant shock channel template of FIG.16 in greater detail.

[0034] While the invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail hereinbelow. It is to beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

[0035] In the following description of the illustrated embodiments,references are made to the accompanying drawings which form a parthereof, and in which is shown by way of illustration, variousembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized, and structural andfunctional changes may be made without departing from the scope of thepresent invention.

[0036] The embodiments of the present system illustrated herein aregenerally described as being implemented in an implantable cardiacdefibrillator, which may operate in numerous pacing modes known in theart. The systems and methods of the present invention may also beimplemented in other implantable medical devices that sense cardiacactivity, such as pacemakers and cardiac monitors, for example.

[0037] In one embodiment, an implantable cardiac defibrillatorconfigured as a single chamber defibrillator operates to generate asnapshot representative of one beat of a patient's normal cardiac rhythmin accordance with the principles of the present invention. In anotherembodiment, an implantable cardiac defibrillator that incorporates thesystems and methods of the present invention is a dual chamberdefibrillator. Various types of single and multiple chamber implantablecardiac defibrillators are known in the art.

[0038] The systems and methods of the present invention may also beimplemented in external cardioverter/monitor systems as are known in theart. Also, the present medical system can also be implemented in animplantable atrial cardioverter/defibrillator, which may includenumerous pacing modes known in the art. Furthermore, although thepresent system is described in conjunction with an implantable cardiacdefibrillator having a microprocessor-based architecture, it will beunderstood that the implantable cardiac defibrillator (or otherimplanted device) may be implemented in any logic-based integratedcircuit architecture, if desired.

[0039] The present invention provides for systems and methods formonitoring a patient's electrocardiogram and producing a snapshotrepresentative of one of the patient's normal supra-ventricularconducted beats. Producing such a snapshot may be effected at any timefor a number of different purposes. By way of example, the diagnosis ofa patient's cardiac rhythms may be enhanced by comparing QRS complexesof a current cardiac rhythm to a snapshot of the patient's normalcardiac rhythm produced by employment of the methodologies of thepresent invention. By way of further example, the titration of drugdosage based on electrocardiographic properties of such a snapshotproduced in accordance with the present invention may also be enhanced.

[0040] The methodologies of producing an accurate snapshot of apatient's normal cardiac rhythm may be used in combination with anautomatic VT/SVT (ventricular tachyarrhythmia/supra-ventriculartachyarrhythmia) rhythm discrimination technique employed in animplantable cardioverter/defibrillator (ICD). Also, the methodologies ofthe present invention may be used as a component of an automatic Holteranalysis system employed in an implantable pacemaker, for example. Theseand other applications may be enhanced by employment of the systems andmethods of the present invention.

[0041] Referring now to FIG. 1 of the drawings, there is shown oneembodiment of a medical device system which includes an implantablecardiac defibrillator 100 electrically and physically coupled to anintracardiac lead system 102. The intracardiac lead system 102 isimplanted in a human body with portions of the intracardiac lead system102 inserted into a heart 106. The intracardiac lead system 102 is usedto detect and analyze electric cardiac signals produced by the heart 106and to provide electrical energy to the heart 106 under certainpredetermined conditions to treat cardiac arrhythmias, including, forexample, ventricular fibrillation of the heart 106. In an embodiment inwhich only monitoring of cardiac activity is performed, the intracardiaclead system 102 need not provide for the production of electrical energyto stimulate the heart 106.

[0042] The intracardiac lead system 102 includes one or more pacingelectrodes and one or more intracardiac defibrillation electrodes. Inthe particular embodiment shown in FIG. 1, the intracardiac lead system102 includes a ventricular lead system 104 and an atrial lead system106. The ventricular lead system 104 includes an SVC-coil 116, anRV-coil 114, and an RV-tip electrode 112. The RV-coil 114, which is alsoreferred to as an RV-ring electrode, is spaced apart from the RV-tipelectrode 112, which is a pacing electrode. In one embodiment, theventricular lead system 104 is configured as an integrated bipolarpace/shock lead.

[0043] The atrial lead system 106 includes an A-tip electrode 152 and anA-ring electrode 154. In one embodiment, the atrial lead system 106 isconfigured as an atrial J lead.

[0044] In this configuration, the intracardiac lead system 102 ispositioned within the heart 106, with a portion of the atrial leadsystem 106 extending into the right atrium 120 and portions of theventricular lead system 104 extending into the right atrium 120 andright ventricle 118. In particular, the A-tip electrode 152 and A-ringelectrode 154 are positioned at appropriate locations within the rightatrium 120. The RV-tip electrode 112 and RV-coil 114 are positioned atappropriate locations within the right ventricle 118. The SVC-coil 116is positioned at an appropriate location within the right atrium chamber120 of the heart 106 or a major vein leading to the right atrium chamber120 of the heart 106. The RV-coil 114 and SVC-coil 116 depicted in FIG.1 are defibrillation electrodes.

[0045] Additional pacing and defibrillation electrodes can also beincluded on the intracardiac lead system 102 to allow for variousbipolar sensing, pacing, and defibrillation capabilities. For example,the intracardiac lead system 102 may include endocardial pacing andcardioversion/defibrillation leads (not shown) that are advanced intothe coronary sinus and coronary veins to locate the distal electrode(s)adjacent to the left ventricle or the left atrium. The distal end ofsuch coronary sinus leads is advanced through the superior vena cava,the right atrium, the valve of the coronary sinus, the coronary sinus,and into a coronary vein communicating with the coronary sinus, such asthe great vein. Other intracardiac lead and electrode arrangements andconfigurations known in the art are also possible and considered to bewithin the scope of the present system.

[0046] The ventricular and atrial lead systems 104, 106 includeconductors for communicating sense, pacing, and defibrillation signalsbetween the cardiac defibrillator 100 and the electrodes and coils ofthe lead systems 104,106. As is shown in FIG. 1, ventricular lead system104 includes a conductor 108 for transmitting sense and pacing signalsbetween the RV-tip electrode 112 and an RV-tip terminal 202 within thecardiac defibrillator 100. A conductor 110 of the ventricular leadsystem 104 transmits sense signals between the RV-coil or ring electrode114 and an RV-coil terminal 204 within the cardiac defibrillator 100.The ventricular lead system 104 also includes conductors 122,124 fortransmitting sense and defibrillation signals between terminals 206, 208of the cardiac defibrillator 100 and SVC- and RV-coils 116 and 114,respectively. The atrial lead system 106 includes conductors 132, 134for transmitting sense and pacing signals between terminals 210, 212 ofthe cardiac defibrillator 100 and A-tip and A-ring electrodes 152 and154, respectively.

[0047] Referring now to FIG. 2, there is shown an embodiment of acardiac defibrillator 100 suitable for implementing a normal cardiacrhythm template generation methodology of the present invention. Thecardiac defibrillator 100 includes control system circuitry 101 forreceiving cardiac signals from a heart 106 and delivering electricalenergy to the heart 106. The control system circuitry 101 includesterminals 202, 204, 206, 208, 209, 210, and 212 for connecting to theelectrodes and coils of the intracardiac lead system 102, as previouslydiscussed.

[0048] In one embodiment, the control system circuitry 101 of thecardiac defibrillator 100 is encased and hermetically sealed in ahousing 130 suitable for implanting in a human body as is known in theart. Power to the cardiac defibrillator 100 is supplied by anelectrochemical battery 256 that is housed within the cardiacdefibrillator 100. A connector block (not shown) is additionallyattached to the housing 130 of the cardiac defibrillator 100 to allowfor the physical and electrical attachment of the intracardiac leadsystem conductors to the cardiac defibrillator 100 and the encasedcontrol system circuitry 101.

[0049] In one embodiment, the control system circuitry 101 of thecardiac defibrillator 100 is a programmable microprocessor-based system,with a controller 216 and a memory circuit (not shown). The memorycircuit contains parameters for various pacing, defibrillation, andsensing modes and stores data indicative of cardiac signals received bythe control system circuitry 101. The controller 216 and memory circuitcooperate with other components of the control system circuitry 101 toperform operations involving the generation of a template representing asnapshot of one beat of a patient's normal cardiac rhythm according tothe principles of the present invention, in addition to other sensing,pacing and defibrillation functions. A memory 213 is also provided forstoring historical EGM and therapy data, which may be used on-board forvarious purposes and transmitted to an external programmer unit 228 asneeded or desired.

[0050] Telemetry circuitry 224 is additionally coupled to the controlsystem circuitry 101 to allow the cardiac defibrillator 100 tocommunicate with an external programmer unit 228. In one embodiment, thetelemetry circuitry 224 and the programmer unit 228 use a wire loopantenna and a radio frequency telemetric link, as is known in the art,to receive and transmit signals and data between the programmer unit 228and the control system circuitry 101. In this manner, programmingcommands and instructions are transferred to the controller 216 of thecardiac defibrillator 100 from the programmer unit 228 during and afterimplant, and stored cardiac data pertaining to sensed arrhythmicepisodes within the heart 106, template information, and subsequenttherapy or therapies applied to correct the sensed arrhythmic event aretransferred to the programmer unit 228 from the cardiac defibrillator100, for example.

[0051] Cardiac signals sensed through use of the RV-tip electrode 112are near-field signals or rate channel signals as are known in the art.More particularly, a rate channel signal is detected as a voltagedeveloped between the RV-ip electrode 112 and the RV-coil 114. Ratechannel signals developed between the RV-tip electrode 112 and theRV-coil 114 are referred to herein as rate channel signals or signalsmeasured from the rate channel.

[0052] Cardiac signals sensed through use of one or both of thedefibrillation coils or electrodes 114, 116 are far-field signals, alsoreferred to as morphology or shock channel signals, as are known in theart. More particularly, a shock channel signal is detected as a voltagedeveloped between the RV-coil 114 and the SVC-coil 116. A shock channelsignal may also be detected as a voltage developed between the RV-coil114 and the SVC-coil 116 and can electrode 209. A shock channel signalmay further be detected as a voltage developed between the RV-coil 114and the can electrode 209. Shock channel signals developed usingappropriate combinations of the RV-coil, SVC-coil, and can electrodes114, 116 and 209 are sensed and amplified by a shock EGM amplifier 238,the output of which is coupled to the tachyarrythmia detector 250.

[0053] In the embodiment of the cardiac defibrillator 100 depicted inFIG. 2, RV-tip and RV-coil electrodes 112, 114 are shown coupled to aV-sense amplifier 230. Rate channel signals received by the V-senseamplifier 230 are communicated to an R-wave detector 236. The R-wavedetector 236 serves to sense and amplify the rate channel signals (e.g.,R-waves) and communicate the detected signals to a pacemaker 240 and atachyarrythmia detector 250.

[0054] A-tip and A-ring electrodes 152,154 are shown coupled to anA-sense amplifier 220. Atrial sense signals received by the A-senseamplifier 220 are communicated to an A-wave detector 222, which servesto sense and amplify the A-wave signals. The atrial signals arecommunicated from the A-wave detector 222 to the pacemaker 240 and thetachyarrythmia detector 250. The pacemaker 240 communicates pacingsignals to the RV-tip and A-tip electrodes 112 and 152 according to apreestablished pacing regimen under appropriate conditions. Blankingcircuitry (not shown) is employed in a known manner when a ventricularor atrial pacing pulse is delivered, such that the ventricular channel,atrial channel, and shock channel are properly blanked at theappropriate time and for the appropriate duration.

[0055] The cardiac defibrillator 100 depicted in FIG. 1 is well-suitedfor implementing a template generation methodology according to theprinciples of the present invention. In the embodiment show in FIG. 1,the template generation processes of the present invention are carriedout by the tachyarrhythmia detector/template generator 250. The shockchannel and rate channel signals used for template generation andrelated template operations are provided by the shock EGM amplifier 238and the V-sense amplifier 230, respectively. It is understood that therequired shock and rate channel signals may be developed and processedby components other than those depicted in FIG. 1 for systemarchitectures that differ from that described herein.

[0056] In general terms, a template refers to a set of points, calledfeatures, which describes a representative waveform of atrial origin asmeasured from the shock channel, together with the predominate fiducialpoint polarity of the same waveform. The fiducial point is derived fromthe rate channel.

[0057] The number of points or features that define a template ispreferably a set of eight points, but may vary. The features are used tocompare other template beats with the reference template. The morecorrelated a beat is with the template, the higher the likelihood thatthe beat is of atrial origin. It is noted that the term “correlate” inthis context means that a feature correlation coefficient (FCC), thesquare of the correlation coefficient, exceeds a given constant. It isfurther noted that the processes and calculations discussed herein donot imply a specific design, hardware or software architecture orimplementation.

[0058] The template is updated periodically after initial templategeneration. When a template update is initiated, the current rhythm ischecked for rate and beat regularity. If the rhythm rate and regularitysatisfy certain criteria, the stored template is checked for correlationwith the current template beats. If the current SVR (supra-ventricularrhythm) morphology has changed sufficiently from that of the storedtemplate, a new candidate template is generated to potentially replacethe stored template, as will be discussed below in greater detail.

[0059] Turning now to FIG. 3, there is illustrated various processesinvolving the production of a snapshot representative of one beat of apatient's normal cardiac rhythm according to an embodiment of thepresent invention. A template is generated through multiple stages andmay be regenerated or updated periodically as needed or desired. Uponinitiation 300 of template operations, rate channel signals, whichconstitute near-field signals, are sensed 302. Shock channel signals,which constitute far-field signals, are also sensed 304. A fiducialpoint for the rate channel signals is determined 306. The shock channelwaveforms are then aligned 308 using the fiducial point developed fromthe rate channel signals. A template is generated 310 using the alignedshock channel waveforms. The template generation procedure is thencompleted 312. The template may be updated 314 periodically as needed ordesired, which involves comparing the currently stored template withsubsequently received template beats on a beat-by-beat basis.

[0060]FIG. 4 illustrates various processes of an automatic templateupdate procedure in accordance with an embodiment of the presentinvention. According to this embodiment, upon initiation 400 of atemplate update procedure, the rate and regularity of sensed R-waves arecalculated 402. During the template update procedure, the rate andregularity are repeatedly calculated and checked for “normalcy” withrespect to predefined criteria, as will be described in greater detailbelow.

[0061] For example, after the initial rate and regularity computationsare performed at the 20^(th) RR interval, the RR average interval andrate regularity are continuously calculated for every beat during thetemplate update procedure. If the rate and regularity are acceptable404, a check is made to determine 406 whether there exists a storedtemplate. If not, a candidate template is generated 408. The currentrhythm is compared with the candidate template. If the current rhythmcorrelates with the candidate template 410, the candidate template isstored 412 as the current template. If the current rhythm does notcorrelate with the candidate template 410, the candidate template isdiscarded and the template update procedure is terminated 424 andsubsequently reinitiated in accordance with programming.

[0062] If there exists a stored template 406, a check is made todetermine if the current rhythm correlates with the stored template 414.If the current rhythm correlates with the stored template, the storedtemplate is retained 422 and the template update procedure is terminated424 and subsequently reinitiated in accordance with programming. If,however, the current rhythm does not correlate with the stored template414, a candidate template is generated 416. If the current rhythmcorrelates with the candidate template 418, the stored template isreplaced 420 with the newly generated candidate template, and thetemplate update procedure is terminated 424 and subsequently reinitiatedin accordance with programming. If, however, the current rhythm does notcorrelate with the candidate template 418, the currently stored templateis retained 422, and the template update procedure is terminated 424 andsubsequently reinitiated in accordance with programming.

[0063] Referring now to FIG. 5, a number of additional templategeneration processes will now be described. FIG. 6 depicts these andother template generation processes in a linear time fashion accordingto an embodiment of the present invention. Rate and shock channelsignals are sensed 500, 502 in a manner described previously.

[0064] Template update operations are initiated 504 and terminatedaccording to programming and under certain conditions. The templateupdate time period is programmable, such as in a range of 10 minutes to24 hours, with 10-minute increments, for example. The nominal value is120 minutes. The update time period is typically not a user programmableparameter.

[0065] A template update is initiated 504 when one of several initiatingevents occurs. For example, a template update may be initiated inresponse to certain mode switching, such as when the cardiacdefibrillator is programmed from Off mode to Monitor orMonitor-plus-Therapy mode, or from Monitor mode to Monitor-plus-Therapymode, or tachyarrhythmia discrimination programming is programmed fromOFF to ON. A template update may be initiated upon detectingconnectivity between the cardiac defibrillator and implanted leads whenthe leads are connected to the defibrillator. In a preferred embodiment,a template update can be initiated by the clinical user via an externalprogrammer.

[0066] A template update is also initiated when a scheduled update timearrives and the previous update is finished. An update timer restartswhen one of several events occurs. A template update is manuallyinitiated by changing the mode as described in the preceding paragraph.A template update is initiated in response to expiration of the updatetimer.

[0067] A template update is immediately aborted under certainconditions, such as: onset of ventricular tachycardia (VT); expirationof the update timer while the VT is active; or the update timer expireswhile the post therapy timer is not expired. For example, a templateupdate is aborted in response to delivery of a therapy. Such therapiesinclude, for example, any induction attempt or tachyarrythmia therapydelivery, such as Fib Hi, Fib Lo, Shock on T, Ventricular PES,Ventricular burst pacing, Ventricular ATP, and Ventricular therapyshock.

[0068] After initiating 504 a template update, heart rate and regularityare checked 506, 508. RR intervals are developed from the sensed ratechannel signals. An RR interval is measured as an interval between Vs toVs, Vs to Vp, Vp to Vs, or Vp to Vp events, where Vs is the ventricularsensed event detection time and Vp is the ventricular pace pulsedelivery time.

[0069] The initial RR average (RRavg) is calculated as the average ofthe first four RR intervals. The RRavg is calculated as a runningaverage as is characterized in Equation [1] below:

RRavg(l)=0.875*RRavg(l−1)+0.125*RR(l)  [1]

[0070] where, RR(l) is the current RR interval and RRavg(0) is theinitial RR average.

[0071] When the 20th RR interval after the initial RR averagecomputation is acquired, RRavg(20) is compared to a rate threshold. Ifthe heart rate is too fast, then the template update is suspended untilthe next scheduled template update time. According to one configuration,the rate is defined as too fast if RRavg is less than an intervalcorresponding to the smaller of 110 bpm or 5 bpm below the lowesttachyarrhythmia threshold.

[0072] A beat is classified as a “regular” beat when an RR interval islarger than 87.5% and less than 125% of the RRavg. The first regularbeat is available only after initial RRavg is calculated.

[0073] Heart rate is classified as “regular” if at least 50% of thebeats are regular. According to one approach, after the 20^(th) RRinterval is acquired, heart rate regularity is checked. If the rate isnot regular, the template update is suspended until the next scheduledtemplate update time.

[0074] After initial rate and regularity computations are completed atthe 20th RR interval, the RRavg and rate regularity are continuouslycalculated for every beat during the template update procedure. A 20 RRinterval moving window is used when rate regularity is continuouslycalculated. If the rate becomes too high or the rate becomes irregularat any stage of the template update procedure, the template update issuspended immediately and reinitiated at the next update time.

[0075] If a subsequently sensed beat is detected before the analysis ofa current beat is finished, the analysis of the subsequent beat can beoptionally skipped 512. The number of analysis skipped beats of thelatest 20 beats is continuously counted. If the number of analysisskipped beats is greater than 4, then the template update is suspendedimmediately until the next update time. However, it is preferable thatevery RR interval is calculated and used to update the RRavg and rateregularity computations. If RRavg or rate regularity is not updated atany RR interval, the template update is suspended immediately until thenext update time.

[0076] A noise check algorithm is initiated 514 after the initial RRaverage is computed. The noise level, for example, may be measured inthe ST segment or PR segment of the shock channel. By way of example,the PR noise window may be set to 100 ms in duration starting at thefiducial point minus 150 ms. The PR noise level is measured as theabsolute maximum peak value in the PR noise window. If the noise levelis too high, such as greater than 20% of the R-wave peak, for example,then an excessively noisy condition is indicated. The ST noise windowmay be 100 ms in duration starting 100 ms at the fiducial point plus 150ms. The noise level is measured as the number of baseline crossings inthe ST noise window. If the number of baseline crossings is excessivelylarge, such as greater than 5, for example, then an excessively noisycondition is indicated.

[0077] If multiple events of the rate channel are triggered in shortintervals, and the width of the beats exceeds 200 ms, then the ratechannel is classified as noisy. If the rate channel is classified asnoisy, then the beat is classified as a noisy beat.

[0078] A beat satisfying all of the following conditions is classified516 as an NSR beat: 1) the largest amplitude of a beat sensed from therate channel is larger than 50% of the maximum ADC (analog-to-digitalconverter) value; 2) the beat is not a ventricular paced beat and theprevious beat is not a ventricular paced beat; 3) the beat is a regularbeat as defined hereinabove; 4) the beat is not noisy as definedhereinabove; and 5) the beat is not an analysis skipped beat as definedhereinabove.

[0079] An automatic gain control (AGC) check is performed 518 on theshock channel. The shock channel AGC procedure involves measuring theamplitude of an NSR beat sensed from the shock channel from a windowstarting 100 ms before Vs, with a duration of 200 ms if there is noatrial pacing pulse within an applicable tachyarrhythmia discriminationdetection window. When the 20^(th) beat is acquired for rate and rateregularity computations, the average peak is computed.

[0080] If the number of NSR beats is less than 11, the update issuspended until the next scheduled template update time. The shockchannel gain is adjusted to an available gain that sets the average peaknearest to 60% of the maximum ADC value. For example, if the averagepeak amplitude of the NSR beats does not fall within a specified range,such as 30% to 70% of a specified maximum value, then the gain of theshock channel is adjusted to an available gain that sets the averagepeak amplitude nearest to 60% of the maximum ADC value.

[0081] An NSR beat that meets all of the following additional conditionsis classified 520 as a template beat that is used to form a new templateor to confirm a template. Template beats are classified only if there isa defined fiducial point type. The additional conditions are: 1) theamplitude of the fiducial point is larger than 50% of the maximum ADCvalue; 2) the saturated fiducial point (i.e., a point with either themaximum positive or maximum negative ADC value) is not followed byanother saturated sample on the rate channel; 3) the shock channel beatamplitude is not less than 40% or greater than 90% of the maximum ADCvalue; 4) the following Vs is not detected within the applicabletachyarrythmia discrimination detection window; and 5) an atrial pacingpulse does not occur within the applicable tachyarrythmia discriminationdetection window.

[0082] After shock channel AGC is performed, and if there are storedtemplate features, the currently stored template features are checked522 prospectively with newly detected template beats. This is abeat-by-beat operation, and there is no need to store multiple beats.

[0083] The fiducial point type of the current template features is usedfor time alignment. If at least 10 beats out of 21 template beats haveFCC values larger than a preestablished FCC threshold (e.g., an FCCvalue of 0.95), then the currently stored template features aresufficiently representative of the template beats, and the templateupdate is suspended until the next scheduled template update time.Otherwise, generation of new template features is attempted to replacethe currently stored template features. If it is required to collectmore than 50 analyzed beats to obtain 21 template beats, the update isaborted until the next scheduled template update time.

[0084] As discussed previously, a template is generated using a fiducialpoint developed from rate channel signals for purposes of shock channelwaveform alignment. The fiducial point type is either positive (Pos) ornegative (Neg). The positive peak (Pos) and negative peak (Neg) of asensed beat detected on the rate channel determines the fiducial pointtype. FIGS. 7 and 8 depict positive and negative fiducial point types,respectively. The Pos and Neg peaks are measured as absolute values. Foreach NSR beat, the positive peak (Pos) and negative peak (Neg) aremeasured from a window starting at Vs, with a duration of 100 ms. Thefiducial point type of a beat is determined as follows:

[0085] If Pos>0.9 * Neg, the fiducial point type is Pos

[0086] Otherwise, the fiducial point type is Neg

[0087] After shock channel AGC is performed, the fiducial point type ofeach NSR beat is evaluated. After 21 fiducial point types are evaluated,majority rule is applied to determine the fiducial point type foralignment. If it takes more than 50 analyzed beats to acquire 21fiducial types, the template update is suspended until the nextscheduled template update time.

[0088] The shock channel waveforms of template beats are aligned withinthe aforementioned tachyarrhythmia discrimination detection window usingthe new fiducial point developed from rate channel signals. In oneembodiment, and as depicted in FIG. 9, the tachyarrhythmiadiscrimination detection window consists of 65 samples centered at thefiducial point which are used for template generation. The templatewaveform is generated using point-by-point averaging of 16 templatebeats. In particular, 16 samples acquired from the same time location ofaligned 16 template beats are averaged or median filtered to generate asample of the template waveform. For example, the template waveform(i)may be characterized by Equation [2] below: $\begin{matrix}{{{Template}\quad {{Waveform}(i)}} = {\frac{1}{16}{\sum\limits_{j = 1}^{16}\quad {{Template}\quad {{Beat}\left( {i,j} \right)}}}}} & \lbrack 2\rbrack\end{matrix}$

[0089] where, the term template beat (i,j) is the i^(th) sample from thedetection window of the j^(th) template beat. If more than 50 analyzedbeats are required to obtain 16 template beats, the template update isaborted and rescheduled.

[0090] According to an embodiment of the present invention, and withreference to FIGS. 10 and 11, five features are initially selected forthe shock channel template, followed by three additional featuresdetermined at midpoints between certain ones of the five selectedfeatures. The five features of the template are determined in thefollowing manner.

[0091] Feature 3 is selected as the absolute maximum peak in a featurewindow defined by 31 samples centered at the fiducial point. If thepositive peak amplitude is equal to the negative peak amplitude, thepositive peak is selected as Feature 3.

[0092] Feature 2 is found by searching backward from Feature 3 until apoint is reached that meets the following conditions: 1) the search islimited to 10 samples. If no point satisfies the following conditions,then the 10th sample becomes Feature 2; 2) the amplitude is less than25% of the maximum peak; and 3) a turning point is found or the slope isflat.

[0093] By way of example, let Q(l) represent the current sample. Aturning point is found if:

[0094] Q(l−1)>Q(l) and Q(l)<Q(l+1) for a positive Feature 3

[0095] Q(l−1)<Q(l) and Q(l)>Q(l+1) for a negative Feature 3

[0096] As is shown in FIG. 10, Q(l−1) is selected as Feature 2. As such,Feature 2 is selected as a turning point.

[0097] The slope is considered flat, as shown in FIG. 11, ifabs(Q(l)−Q(l−11))<4 and abs(Q(l)−Q(l−2))<4, in the case when the ADCmaximum value is 128. In the illustrative depiction of FIG. 11, Q(l−1)is selected as Feature 2. As such, Feature 2 is selected as a flat slopepoint.

[0098] Feature 4 is found by searching forward starting from Feature 3until a point is reached that meets the following conditions: 1) thesearch is limited to 10 samples. If no point satisfies the followingconditions, then the 10th sample becomes Feature 4; 2) the amplitude isless than 25% of the maximum peak; and 3) a turning point is found orthe slope is flat.

[0099] By way of example, let Q(l) represent the current sample. Aturning point is found if:

[0100] Q(l+1)>Q(l) and Q(l)<Q(l−1) for a positive Feature 3

[0101] Q(l+1)<Q(l) and Q(l)>Q(l−1) for a negative Feature 3

[0102] Q(l+1) is selected as Feature 4, as is shown in FIG. 10. Theslope is flat if abs(Q(l)−Q(l+1))<4 and abs(Q(l)−Q(l+2))<4. In thiscase, Q(l+1) is selected as Feature 4.

[0103] Feature 1 is selected as the seventeenth sample from thebeginning of the detection window. Feature 5 is selected as the lastsample of the detection window. Three additional features are selectedat the midpoint of Features 1 and 2, the midpoint of Features 2 and 3,and the midpoint of Features 3 and 4, respectively. If a midpoint fallsbetween two sample points, the leftmost (earlier in time) point isselected. Thus, according to this embodiment, eight feature values(e.g., amplitudes) and their associated locations with respect to thefiducial point and the corresponding fiducial point type are saved forbeat classification.

[0104] The new template features are confirmed prospectively with newlydetected template beats. As discussed previously, this confirmationprocesses is performed on a beat-by-beat basis, such that there is noneed to store template data for multiple beats.

[0105] In one particular embodiment, Equation [3], provided below, isused to compute the feature correlation coefficient (FCC) betweentemplate features and beat features to be classified: $\begin{matrix}{{FCC} = \frac{\left( {{N{\sum\limits_{i = 1}^{N}\quad {X_{i}Y_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}\quad X_{i}} \right)\left( {\sum\limits_{i = 1}^{N}\quad Y_{i}} \right)}} \right)^{2}}{\left( {{N{\sum\limits_{i = 1}^{N}\quad X_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\quad X_{i}} \right)^{2}} \right)\left( {{N{\sum\limits_{i = 1}^{N}\quad Y_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\quad Y_{i}} \right)^{2}} \right)}} & \lbrack 3\rbrack\end{matrix}$

[0106] where, Xi represents template N features and Yi represents beat Nfeatures, and N=8 in this illustrative example. The sign of thenumerator term${N{\sum\limits_{i = 1}^{N}\quad {X_{i}Y_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}\quad X_{i}} \right)\left( {\sum\limits_{i = 1}^{N}\quad Y_{i}} \right)}$

[0107] is checked before squaring. If the numerator is negative, thebeat is uncorrelated, and the remainder of the computation need not beperformed.

[0108] If at least 10 beats out of 21 template beats have an FCC greaterthan the FCC threshold (e.g., 0.95), the new template features replacethe currently stored template features. Otherwise, the new templatefeatures do not sufficiently represent the new template beats, and thecurrently stored template features are kept until the next scheduledtemplate update time. If it is required to collect more than 50 analyzedbeats to obtain 21 template beats, the template update is suspendeduntil the next scheduled template update time.

[0109] For purposes of illustration, and not of limitation, FIGS. 12-16show shock and rate channel waveforms associated with various stages ofa template generation methodology of the present invention. In FIG. 12,there is illustrated a shock channel template, shown as FIG. 12A, and arate channel waveform of a positive fiducial point type, shown as FIG.12B. The shock and rate waveforms depicted in FIGS. 12A and 12B aredeveloped in a manner previously described.

[0110]FIGS. 13A and 13B reiterate the shock and rate waveforms of FIGS.12A and 12B, respectively. In addition, FIG. 13A illustrates fivefeatures that have been initially selected for performing FCCcomputations. According to this embodiment, feature F₃ of the shockchannel template shown in FIG. 13A is selected as the absolute maximumpeak. Features F₂ through F₅ are selected in a manner previouslydiscussed. Intermediate features may also be computed, but are not shownin FIGS. 13A or 14A. For example, a total of eight features may be usedfor performing FCC computations.

[0111] When performing FCC computations, the amplitudes associated withlocations of the selected features are stored. For example, and withreference to FIG. 14A in particular, the stored locations associatedwith the five features, F₁-F_(5,) are given as locations [3, 25, 29, 36,65]. The amplitudes associated with these locations are given asamplitudes [0, 5, 50, −50, 0]. These amplitudes for the features of thecurrently stored shock channel template and those of the newly detectedtemplate beats are used for performing FCC computations associated withan automatic shock channel template update procedure.

[0112] As discussed previously, and to summarize by use of thedepictions in FIGS. 15 and 16, two channels of continuous egram areemployed by the template generation algorithm of the present invention.More particularly, a rate channel and a shock or morphology channel areemployed. The template generation algorithm preferably activates on aperiodic basis, such as every two hours. It is noted that the wake-upduration is programmable and may be variable. The template generationalgorithm performs the following operations.

[0113] If a previous template exists and continues to qualify as asnapshot representative of one beat of a patient's normal cardiacrhythm, then the previous template is retained as the current template.Otherwise, an attempt is made to generate a new template. FIGS. 15 and16 illustrate various stages of template generation in accordance withan embodiment of the present invention.

[0114]FIG. 15 depicts a template waveform generated using a detectionwindow consisting of 65 samples centered at the fiducial point (i.e.,point 32). FIG. 16 illustrates a shock channel template generated by theaforementioned averaging process, with a number of features selected forthe template also being shown. The features are selected in a mannerdescribed herein.

[0115] Template generation in accordance with the principles of thepresent invention provides for several advantages. For example, thetemplate generation methodology of the present invention requires onlybeat-by-beat analysis and is well-suited for use in implantable devices,such as in implantable cardioverter/defibrillator devices. Moreover, themulti-stage approach of the present invention is efficient in its memoryusage. Further, the template generation approach of the presentinvention is robust in generating a snapshot representative of one beatof a patient's normal cardiac rhythm in the presence of prematureventricular complexes (PVCs). The template generation approach of thepresent invention is well-suited for use in conjunction with a VT/SVTrhythm discrimination system.

[0116] It will, of course, be understood that various modifications andadditions can be made to the preferred embodiments discussed hereinabovewithout departing from the scope of the present invention. Accordingly,the scope of the present invention should not be limited by theparticular embodiments described above, but should be defined only bythe claims set forth below and equivalents thereof.

What is claimed is:
 1. A method of generating a snapshot representativeof one beat of a patient's normal cardiac rhythm, comprising: sensingrate channel signals; sensing shock channel signals; determining afiducial point for the rate channel signals; aligning the shock channelsignals using the fiducial point; and generating a template using thealigned shock channel signals, the template representative of one of thepatient's normal supra-ventricular conducted beats.
 2. The method ofclaim 1, further comprising updating the template on a periodic basis.3. The method of claim 1, further comprising confirming, usingsubsequently detected template beats, whether the generated template isor is not representative of one of the patient's normalsupra-ventricular conducted beats.
 4. The method of claim 1, furthercomprising determining that no template is presently stored, and storingthe generated template in response to confirming that subsequentlydetected template beats correlate with the generated template.
 5. Themethod of claim 1, further comprising determining that no template ispresently stored, and discarding the generated template in response toconfirming that subsequently detected template beats fail to correlatewith the generated template.
 6. The method of claim 1, furthercomprising: determining that the generated template is stored; andretaining the stored template in response to determining thatsubsequently detected template beats correlate with the stored template.7. The method of claim 1, further comprising: determining that thegenerated template is stored; generating a new template in response toconfirming that subsequently detected template beats fail to correlatewith the stored template; and replacing the stored template with the newtemplate in response to confirming that newly detected template beatscorrelate with the new template.
 8. The method of claim 1, furthercomprising: determining that the generated template is stored;generating a new template in response to the confirming thatsubsequently detected template beats fail to correlate with the storedtemplate; and retaining the stored template and discarding the newtemplate in response to confirming that newly detected template beatsfail to correlate with the new template.
 9. The method of claim 1,wherein generating the template comprises averaging the aligned shockchannel signals.
 10. The method of claim 9, wherein averaging thealigned shock channel signals comprises point-by-point averaging ormedian filtering n samples acquired from the same time location ofaligned n template beats.
 11. The method of claim 1, further comprisesdetermining that the rate channel signals satisfy predefined normalcycriteria using a running average (RRavg) of a plurality of RR intervals.12. The method of claim 1, further comprising: initiating templateupdating; comparing a running average (RRavg) of a plurality of RRintervals to a predetermined rate threshold; and suspending templateupdating if RRavg is less than a predetermined interval.
 13. The methodof claim 1, further comprising: computing a running average (RRavg) of aplurality of RR intervals; and classifying a beat as a regular beat ifan RR interval associated with the beat falls within a predeterminedpercentage range of RRavg.
 14. The method of claim 13, furthercomprising classifying a heart rate as regular if a predeterminedpercentage of the beats are regular beats.
 15. The method of claim 1,further comprising skipping processing of a subsequently sensed ratechannel signal if the subsequently sensed rate channel signal isdetected before processing of a current sensed rate channel signal iscompleted.
 16. The method of claim 1, further comprising determiningwhether the rate channel is noisy, and classifying a beat as noisy inresponse to the rate channel being determined noisy.
 17. The method ofclaim 1, further comprising: computing an average peak amplitude of aplurality of beats; and adjusting shock channel gain to an availablegain that sets the average peak amplitude to a predetermined percentageof a maximum ADC value.
 18. The method of claim 1, further comprisingclassifying sensed beats as NSR beats in response to satisfying a firstset of criteria, and classifying the NSR beats as template beats inresponse to satisfying a second set of criteria.
 19. The method of claim18, wherein generating the template further comprises generating thetemplate using the aligned template beats.
 20. The method of claim 1,wherein the fiducial point is characterized by a fiducial point type,the fiducial point type being determined by determining the larger of apositive peak and a negative peak for each of a plurality of NSR beats.21. The method of claim 20, wherein the fiducial point type foralignment is determined by determining if the majority of NSR beats havepositive peaks or negative peaks.
 22. The method of claim 1, whereinaligning the shock channel signals comprises aligning shock channelwaveforms of template beats centered with respect to the fiducial point.23. The method of claim 21, wherein generating the template furthercomprises generating a template waveform by averaging a predeterminednumber of template beats.
 24. The method of claim 1, wherein generatingthe template further comprises determining a plurality of features ofthe template.
 25. The method of claim 24, wherein the plurality oftemplate features comprises an absolute maximum peak and at least one ofa turning point and a flat slope point.
 26. A body implantable system,disposed in a housing having a can electrode, for generating a snapshotrepresentative of one beat of a patient's normal cardiac rhythm,comprising: a lead system comprising electrodes, the lead systemextending from the housing into a heart; a detector system, coupled tothe lead system, that detects rate channel signals and shock channelsignals sensed by one or both of the lead system electrodes and the canelectrode; and a control system coupled to the detector system, thecontrol system determining a fiducial point for the rate channelsignals, aligning the shock channel signals using the fiducial point,and generating a template using the aligned shock channel signals, thetemplate representative of one of the patient's normal supra-ventricularconducted beats.
 27. The system of claim 26, wherein the control systemupdates the template on a periodic basis.
 28. The system of claim 26,wherein the control system confirms whether the generated template is oris not representative of one of the patient's normal supra-ventricularconducted beats using subsequently detected template beats.
 29. Thesystem of claim 26, wherein the control system, in response todetermining that no template is stored in memory, stores the generatedtemplate in memory in response to confirming that subsequently detectedtemplate beats correlate with the generated template.
 30. The system ofclaim 26, wherein the control system, in response to determining that notemplate is stored in memory, discards the generated template inresponse to determining that subsequently detected template beats failto correlate with the generated template.
 31. The system of claim 26,wherein the generated template is stored in memory, the control systemretaining the stored template in memory in response determining thatsubsequently detected template beats correlate with the template storedin memory.
 32. The system of claim 26, wherein the generated template isstored in memory, the control system further generating a new templatein response to confirming that subsequently detected template beats failto correlate with the template stored in memory, the control systemreplacing the template stored in memory with the new template inresponse to confirming that newly detected template beats correlate withthe new template.
 33. The system of claim 26, wherein the control systemstores the generated template in memory, the control system furthergenerating a new template in response to confirming that subsequentlydetected template beats fail to correlate with the template stored inmemory, the control system retaining the template stored in memory anddiscarding the new template in response to confirming that newlydetected template beats fail to correlate with the new template.
 34. Thesystem of claim 26, wherein the control system performs an averagingcomputation on the aligned shock channel signals.
 35. The system ofclaim 26, wherein the control system computes a running average (RRavg)of a plurality of RR intervals and determines that the rate channelsignals satisfy predefined normalcy criteria using RRavg.
 36. The systemof claim 26, wherein the control system initiates template updating,compares a running average (RRavg) of a plurality of RR intervals to apredetermined rate threshold, and suspends template updating if RRavg isless than a predetermined interval.
 37. The system of claim 26, whereinthe control system computes a running average (RRavg) of a plurality ofRR intervals, and classifies a beat as a regular beat if an RR intervalassociated with the beat falls within a predetermined percentage rangeof RRavg.
 38. The system of claim 37, wherein the control systemclassifies a heart rate as regular if a predetermined percentage of thebeats are regular beats.
 39. The system of claim 26, wherein the controlsystem skips processing of a subsequently sensed rate channel signal ifthe subsequently sensed rate channel signal is detected beforeprocessing of a current sensed rate channel signal is completed.
 40. Thesystem of claim 26, wherein the control system determines whether therate channel is noisy, and classifies a beat as noisy in response to therate channel being determined noisy.
 41. The system of claim 26, whereinthe control system computes an average peak amplitude of a plurality ofbeats, and adjusts shock channel gain to an available gain that sets theaverage peak amplitude to a predetermined percentage of a maximum ADCvalue.
 42. The system of claim 26, wherein the control system classifiessensed beats as NSR beats in response to satisfying a first set ofcriteria, and classifies the NSR beats as template beats in response tosatisfying a second set of criteria.
 43. The system of claim 42, whereinthe control system generates the template using the aligned templatebeats.
 44. The system of claim 26, wherein the control system determinesthe fiducial point type by determining the larger of a positive peak anda negative peak for each of a plurality of NSR beats.
 45. The system ofclaim 44, wherein the fiducial point type for alignment is determined bythe control system by determining if the majority of NSR beats havepositive peaks or negative peaks.
 46. The system of claim 26, whereinthe control system aligns shock channel waveforms of template beatscentered with respect to the fiducial point.
 47. The system of claim 46,wherein the control system generates a template waveform by averaging apredetermined number of template beats.
 48. The system of claim 26,wherein the control system determines a plurality of features of thetemplate.
 49. The system of claim 26, wherein the control systemdetermines a plurality of template features comprising an absolutemaximum peak and at least one of a turning point and a flat slope point.50. The system of claim 26, wherein the control system updates thetemplate in response to detecting establishment of connectivity betweenthe lead system and the detector system.
 51. A system for generating asnapshot representative of one beat of a patient's normal cardiacrhythm, comprising: means for sensing rate channel signals; means forsensing shock channel signals; means for determining a fiducial pointfor the rate channel signals; means for aligning the shock channelsignals using the fiducial point; and means for generating a templateusing the aligned shock channel signals, the template representative ofone of the patient's normal supra-ventricular conducted beats.
 52. Thesystem of claim 51, further comprising means for updating the templateon a periodic basis.
 53. The system of claim 51, further comprisingmeans for confirming, using subsequently detected template beats,whether the generated template is or is not representative of one of thepatient's normal supra-ventricular conducted beats.
 54. The system ofclaim 51, further comprising means for determining that no template ispresently stored, and means for storing the generated template inresponse to confirming that subsequently detected template beatscorrelate with the generated template.
 55. The system of claim 51,further comprising means for determining that no template is presentlystored, and means for discarding the generated template in response toconfirming that subsequently detected template beats fail to correlatewith the generated template.
 56. The system of claim 51, furthercomprising: means for determining that the generated template is stored;and means for retaining the stored template in response to determiningthat subsequently detected template beats correlate with the storedtemplate.
 57. The system of claim 51, further comprising: means fordetermining that the generated template is stored; means for generatinga new template in response to confirming that subsequently detectedtemplate beats fail to correlate with the stored template; and means forreplacing the stored template with the new template in response toconfirming that newly detected template beats correlate with the newtemplate.
 58. The system of claim 51, further comprising: means fordetermining that the generated template is stored; means for generatinga new template in response to the confirming that subsequently detectedtemplate beats fail to correlate with the stored template; and means forretaining the stored template and discarding the new template inresponse to confirming that newly detected template beats fail tocorrelate with the new template.
 59. The system of claim 51, furthercomprises means for determining that the rate channel signals satisfypredefined normalcy criteria using a running average (RRavg) of aplurality of RR intervals.
 60. The system of claim 51, furthercomprising means for skipping processing of a subsequently sensed ratechannel signal if the subsequently sensed rate channel signal isdetected before processing of a current sensed rate channel signal iscompleted.
 61. The system of claim 51, further comprising: means forcomputing an average peak amplitude of a plurality of beats; and meansfor adjusting shock channel gain to an available gain that sets theaverage peak amplitude to a predetermined percentage of a maximum ADCvalue.
 62. The system of claim 51, wherein the fiducial point ischaracterized by a fiducial point type, the system further comprisingmeans for determining the fiducial point type as the larger of apositive peak and a negative peak for each of a plurality of NSR beats.63. The system of claim 61, wherein the determining means determines thefiducial point type for alignment by determining if the majority of NSRbeats have positive peaks or negative peaks.
 64. The system of claim 51,wherein the aligning means further comprises means for aligning shockchannel waveforms of template beats centered with respect to thefiducial point.
 65. The system of claim 51, wherein the generating meansfurther comprises means for determining a plurality of features of thetemplate.
 66. The system of claim 51, wherein the generating meansfurther comprises means for determining a plurality of template featurescomprising an absolute maximum peak and at least one of a turning pointand a flat slope point.
 67. The system of claim 51, further comprisingmeans for updating the template in response to detecting establishmentof connectivity between the respective sensing means and at least thetemplate generating means.